Conventionally, wells in oil and gas fields are built up by establishing a wellhead housing, and with a drilling blow out preventer stack (BOP) installed, drilling down to produce the well hole whilst successively installing concentric casing strings, which are cemented at the lower ends and sealed with mechanical seal assemblies at their upper ends. In order to convert the cased well for production, a tubing string is run in-through the BOP and a hanger at its upper end landed in the wellhead. Thereafter the drilling BOP stack is removed and replaced by a Christmas tree having one or more production bores containing actuated valves and extending vertically to respective lateral production fluid outlet ports in the wall of the Christmas tree.
This arrangement has involved problems which have, previously, been accepted as inevitable. Thus any operations down hole have been limited to tooling which can pass through the production bore, which is usually no more than five inch diameter, unless the Christmas tree is first removed and replaced by a BOP stack. However this involves setting plugs or valves, which may be unreliable by not having been used for a long time, down hole. The well is in a vulnerable condition whilst the Christmas tree and BOP stack are being exchanged and neither one is in position, which is a lengthy operation. Also, if it is necessary to pull the completion, consisting essentially of the tubing string on its hanger, the Christmas tree must first be removed and replaced by a BOP stack. This usually involves plugging and/or killing the well.
A further difficulty which exists, particularly with subsea wells, is in providing the proper angular alignment between the various functions, such as fluid flow bores, and electrical and hydraulic lines, when the wellhead equipment, including the tubing hanger, Christmas tree, BOP stack and emergency disconnect devices are stacked up. Exact alignment is necessary if clean connections are to be made without damage as the devices are lowered into engagement with one another. This problem is exacerbated in the case of subsea wells as the various devices which are to be stacked up are run down onto guide posts or a guide funnel projecting upwardly from a guide base. The post receptacles which ride down on to the guide posts or the entry guide into the funnel do so with appreciable clearance. This clearance inevitably introduces some uncertainty in alignment and the aggregate misalignment when multiple devices are stacked, can be unacceptably large. Also the exact orientation will depend upon the precise positions of the posts or keys on a particular guide base and the guides on a particular running tool or BOP stack and these will vary significantly from one to another. Consequently it is preferable to ensure that the same running tools or BOP stack are used for the same wellhead, or a new tool or stack may have to be specially modified for a particular wellhead. Further misalignments can arise from the manner in which the guide base is bolted to the conductor casing of the wellhead.
In accordance with the present invention, a wellhead comprises a wellhead housing; a spool tree fixed and sealed to the housing, and having at least a lateral production fluid outlet port connected to an actuated valve; and a tubing hanger landed within the spool tree at a predetermined angular position at which a lateral production fluid outlet port in the tubing hanger is in alignment with that in the spool tree.
With this arrangement, the spool tree, takes the place of a conventional Christmas tree but differs therefrom in having a comparatively large vertical through bore without any internal valves and at least large enough to accommodate the tubing completion. The advantages which are derived from the use of such spool tree are remarkable, in respect to safety and operational benefits.
Thus, in workover situations the completion, consisting essentially of the tubing string, can be pulled through a BOP stack, without disturbing the spool tree and hence the pressure integrity of the well, whereafter full production casing drift access is provided to the well through the large bore in the spool tree. The BOP can be any appropriate workover BOP or drilling BOP of opportunity and does not have to be one specially set up for that well.
Preferably, there are complementary guide means on the tubing hanger and spool tree to rotate the tubing hanger into the predetermined angular position relatively to the spool tree as the tubing hanger is lowered on to its landing. With this feature the spool tree can be landed at any angular orientation onto the wellhead housing and the guide means ensures that the tubing string will rotate directly to exactly the correct angular orientation relatively to the spool tree quite independently of any outside influence. The guide means to control rotation of the tubing hanger into the predetermined angular orientation relatively to the spool tree may be provided by complementary oblique edge surfaces one facing downwardly on an orientation sleeve depending from the tubing hanger the other facing upwardly on an orientation sleeve carried by the spool tree.
Whereas modern well technology provides continuous access to the tubing annulus around the tubing string, it has generally been accepted as being difficult, if not impossible, to provide continuous venting and/or monitoring of the pressure in the production casing annulus, that is the annulus around the innermost casing string. This has been because the production casing annulus must be securely sealed whist the Christmas tree is fitted in place of the drilling BOP, and the Christmas tree has only been fitted after the tubing string and hanger has been run in, necessarily inside the production casing hanger, so that the production casing hanger is no longer accessible for the opening of a passageway from the production casing annulus. However, the new arrangement, wherein the spool tree is fitted before the tubing string is run in provides adequate protected access through the BOP and spool tree to the production casing hanger for controlling a passage from the production casing annulus.
For this purpose, the wellhead may include a production casing hanger landed in the wellhead housing below the spool tree; an isolation sleeve which is sealed at its lower end to the production casing hanger and at its upper end to the spool tree to define an annular void between the isolation sleeve and the housing; and an adapter located in the annular space and providing part of a passage from the production casing annulus to a production casing annulus pressure monitoring port in the spool tree, the adapter having a valve for opening and closing the passage, and the valve being operable through the spool tree after withdrawal of the isolation sleeve up through the spool tree. The valve may be provided by a gland nut, which can be screwed up and down within a body of the adapter to bring parts of the passage formed in the gland nut and adapter body, respectively, into and out of alignment with one another. The orientation sleeve for the tubing hanger may be provided within the isolation sleeve.
Production casing annulus pressure monitoring can then be set up by method of completing a cased well in which a production casing hanger is fixed and sealed by a seal assembly to a wellhead housing, the method comprising, with BOP installed on the housing, removing the seal assembly and replacing it with an adapter which is manipulatable between configurations in which a passages from the production casing, annulus up past the production casing hanger is open or closed; with the passage closed, removing the BOP and fitting to the housing above the production casing hanger a spool tree having an internal landing for a tubing hanger; installing a BOP on the spool tree; running a tool down through the BOP and spool tree to manipulate the valve and open the passage; inserting through the BOP and spool tree an isolation sleeve, which seals to both the production casing and spool tree and hence defines between the sleeve and casing an annular void through which the passage leads to a production casing annulus pressure monitoring port in the spool tree; and running a tubing string down through the BOP and spool tree until the tubing hanger lands in the spool tree with lateral outlet ports in the tubing hanger and spool tree for production fluid flow, in alignment with one another.
According to a further feature of the invention the spool tree has a downwardly depending location mandrel which is a close sliding fit within a bore of the wellhead housing. The close fit between the location mandrel of the spool tree and the wellhead housing provides a secure mounting which transmits inevitable bending stresses to the housing from the heavy equipment, such as a BOP, which projects upwardly from the top of the wellhead housing, without the need for excessively sturdy connections. The location mandrel may be formed as an integral part of the body of the spool tree, or may be a separate part which is securely fixed, oriented and sealed to the body.
Pressure integrity between the wellhead housing and spool tree may be provided by two seals positioned in series one forming an environmental seal (such as an AX gasket) between the spool tree and the wellhead housing, and the other forming a production seal between the location mandrel and either the wellhead housing or the production casing hanger.
During workover operations, the production casing annulus can be resealed by reversing the above steps, if necessary after setting plugs or packers down hole.
When production casing pressure monitoring is unnecessary, so that no isolation sleeve is required, the orientation sleeve carried by the spool tree for guiding and rotating the tubing hanger down into the correct angular orientation may be part of the spool tree location mandrel itself.
Double barrier isolation, that is to say two barriers in series, are generally necessary for containing pressure in a well. If a spool tree is used instead of a conventional Christmas tree, there are no valves within the vertical production and annulus fluid flow bores within the tree, and alternative provision must be made for sealing the bore or bores through the top of the spool tree which provide for wire line or drill pipe access.
In accordance with a further feature of the invention, at least one vertical production fluid bore in the tubing hanger is sealed above the respective lateral production fluid outlet port by means of a removable plug, and the bore through the spool tree being sealed above the tubing hanger by means of a second removable plug.
With this arrangement, the first plug, takes the function of a conventional swab valve, and may be a wireline set plug. The second plug could be a stopper set in the spool tree above the tubing hanger by, e.g., a drill pipe running tool. The stopper could contain at least one wireline retrievable plug which would allow well access when only wire line operations are called for. The second plug should seal and be locked internally into the spool tree as it performs a barrier to the well when a BOP or intervention module is deployed. A particular advantage of this double plug arrangement is that, as is necessary to satisfy authorities in some jurisdictions, the two independent barriers are provided in mechanically separate parts, namely the tubing hanger and its plug and the second plug in the spool tree.
A further advantage arises if a workover port extends laterally through the wall of the spool tree from between the two plugs; a tubing annulus fluid port extends laterally through the wall of the spool tree from the tubing annulus; and these two ports through the spool tree are interconnected via an external flow line containing at least one actuated valve. The bore from the tubing annulus can then terminate at the port in the spool tree and no wireline access to the tubing annulus bore is necessary through the spool tree as the tubing annulus bore can be connected via the interplug void to choke or kill lines, i.e. a BOP annulus, so that downhole circulation is still available. It is then only necessary to provide wireline access at workover situations to the production bore or bores. This considerably simplifies workover BOP and/or riser construction. When used in conjunction with the plug at the top of the spool tree, the desirable double barrier isolation is provided by the spool tree plug over the tubing hanger, or workover valve from the production flow.
When the well is completed as a multi production bore well, in which the tubing hanger has at least two vertical production through bores each with a lateral production fluid flow port aligned with the corresponding port in the spool tree, at least two respective connectors may be provided for selective connection of a single bore wire line running tool to one or other of the production bores, each connector having a key for entering a complementary formation at the top of the spool tree to locate the connector in a predetermined angular orientation relatively to the spool tree. The same type of alternative connectors may be used for providing wireline or other running tool access to a selected one of a plurality of functional connections, e.g. electrical or hydraulic couplings, at the upper end of the tubing hanger.